Principles of terrestrial ecosystem ecology.pdf

50 3. Geology and Soils
Phosphorus content
Primary P
races have very low productivity. The phenolic
compounds produced by these trees as defenses
against herbivores also retard decomposition,
further reducing soil fertility (Northup et al.
1995) (see Chapter 13).
Potential Biota
Total phosphorus
Occluded
Secondary P P
Organic P
Time
Figure 3.4. The generalized effects of long-term
weathering and soil development on the distribution
and availability of phosphorus (P). Newly exposed
geologic substrate is relatively rich in weatherable
minerals, which release phosphorus. This release
leads to accumulation of both organic and readily
soluble forms (secondary phosphorus, such as
calcium phosphate). As primary minerals disappear
and secondary minerals capable of sorbing phosphorus
accumulate, an increasing proportion of the
phosphorus remaining in the system is held in
unavailable (occluded) forms. Availability of phosphorus
to plants peaks relatively early in this
sequence and declines thereafter. (Redrawn with
permission from Geoderma, Vol. 15 © 1976 Elsevier
Science; Walker and Syers 1976.)
The past and present organisms at a site
strongly influence soil chemical and physical
properties. Most soil development occurs in
the presence of live organisms (Ugolini and
Spaltenstein 1992). There are often clear associations
between vegetation and soils. The
organic acids in the litter of many coniferous
species, for example, acidify the soil. This, in
combination with the characteristically low
quality of conifer litter, leads to slower decomposition
in conifer than in deciduous forests
(Van Cleve et al. 1991) (see Chapter 7). It is
frequently difficult, however, to separate the
chicken from the egg. Did the vegetation determine
soil properties or vice versa?
One approach to determining vegetation
effects on soils has been to plant monocultures
or species mixes into initially homogeneous
sites. Rapidly growing grasses in a nitrogenpoor
perennial grassland enhanced the nitrogen
mineralization of soils within 3 years
(Wedin and Tilman 1990) (see Fig. 12.5), as
did deep-rooted forbs in an annual grassland
(Hooper and Vitousek 1998). Another approach
has been to examine the consequences
of species invasions or extinctions on soil
processes. The invasion of a non-native nitrogen
fixer into Hawaiian rain forests, for
example, increased nitrogen inputs to the
system more than fivefold, altering the characteristics
of soils and the colonization and competitive
balance among native plant species
(Vitousek et al. 1987) (see Fig. 12.3).
Animals also influence soil properties. Earthworms,
termites, and invertebrate shredders
strongly influence decomposition rates (see
Chapter 7) and therefore soil properties that
are influenced by soil organic content.
Human Activities
Since the 1950s, the tripling of the human
population and associated agricultural and
industrial activities have strongly influenced
soil development worldwide. Human activities
influence soils directly through changes in
nutrient inputs, irrigation, alteration of soil
microenvironment through land use change
(see Chapter 14), and increased erosional loss
of soils. Human activities indirectly affect soils
through changes in atmospheric composition
and through the deletions and additions of species.
Today and in the future, human activities
will affect ecosystem properties both directly
and through their effects on other interactive
controls (see Chapters 14 and 16).
Controls over Soil Loss
Soil formation depends on the balance between
deposition, erosion, and soil development. Soil
thickness varies with hillslope position, with